A telecommunication system with high data transmission speed and where data is sent in packets with certain time intervals between the packets, e.g. a system operating with optical signals, necessitates special measures for enabling comprehension and recognition of the value of the bit (a one or a zero) being received. In an optical telecommunication system, e.g. that described in the report "COST 202 Seminar", September 1981, the transmission speed is 100 Mbits/s, implying that the pulse length is 10 ns. Conventional methods in the reception of high speed packets arriving at certain intervals requires clock supervision, which presupposes special coding and associated bandwith loss.